Oxidation of aromatic compounds



OXIDATION OF AROMATIC COMPOUNDS Filed Aug. 31. 1953 Patented Oct. 8, 1957 OXIDATION F AROMATIC COMPOUNDS Art C. McKinnis, North Long Beach, Calif., assigner to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application August 31, 1953, Serial N o. 377,399

11 Claims. (Cl. 260-346.7)

This invention relates to a process for the oxidation of partially oxidized alkyl aromatic hydrocarbons to produce aromatic carboxylic acids. More particularly the invention relates to a continuous process for the oxidation of such partially oxidized alkyl laromatic hydrocarbons to produce aromatic carboxylic `acids in which the carboxyl group or groups are directly attached to nuclear carbon atoms, which process involves the use of air or oxygen and hydrogen sulfide as the only reactants which are consumed in the process. The invention is particularly applicable to the conversion of ortho-toluic acid to phthalic acid or phthalic anhydride.

The oxidation of partially oxidized alkyl substituted aromatic hydrocarbons to produce the corresponding aromatic carboxylic acids is difficult to carry out lbecause if conditions are too severe, oxidation results in rupturing 4the aromatic ring, and if conditions are too mild oxidation takes place at rates so low that the process is not economical or the oxidation may not occur at all. In some instances products intermediate `between the starting materials and the desired carboxylic acids may be obtained. Selecting conditions and oxidizing agents which permit the desired degree of oxidation at reasonable and economical rates which result in high yields of the desired acids has been a goal of investigators in this field.

Some partially oxidized derivatives of alkyl aromatic hydrocarbons can be converted into the corresponding aromatic carboxylic acids by ordinary liquid phase air oxidation. This is particularly true of metaand paratoluic acids alone or in mixtures. The isomeric orthotoluic acid, however, is not oxidized under the same or equivalent conditions and requires somewhat more severe oxidation conditions. Benzyl alcohol and acetophenone also resist oxidation by means of air in the liquid phase as do some of the alkyl substituted benzoic acids. With respect to ortho-toluic acid, it has been found that this compound can be oxidized in the vapor phase over catalysts to produce phthalic anhydride. However, yields are low due to ring cleavage and complete oxidation of part of the ortho-toluic acid charged.

`It has been found that the partial oxidation products of alkyl aromatic hydrocarbons can lbe converted into the corresponding aromatic carboxylic acids in su-bstantially theoretical yields in a continuous process involving recycle of some of the intermediate reaction products, by using as feed to the process only hydrogen sulde and air or oxygen. The process results in the production of the acids without the necessity of using extraneous mineral acids or the like to release the aromatic carboxylic acids from their salts which are initially produced in the process thus avoiding any consumption of bases and acids with the production of inorganic salts as by-products of the process. Materials which are recycled from various stages of the continuous process described herein include hydrogen sulfide, sodium bisulfite and Water.

The process described herein is applicable to the oxidation of substantially any partial oxidation product of an alkyl aromatic hydrocarbon. However, from an economic point of view, aromatic compounds having large alkyl substituents, i. e., of greater length than about 3 carbon atoms and alkyl aromatic hydrocarbons having more than about 3 alkyl groups per molecule, are not suitable. Thus the process is applicable to the partial oxidation products of alkyl aromatic hydrocarbons, such as benzyl alcohol, benzaldehyde, acetophenone, ortho, metaand para-toluic acids, the various ethyl benzoic acids, the various dimethyl benzoic acids, carboxy phenyldimethylcarbinol and like compounds. The process is particularly adaptable to the oxidation of ortho-toluic acid to form phthalic acid or phthalic anhydride.

It is an object of this invention to provide a continuous process for the conversion of partial oxidation products of alkyl aromatic hydrocarbons into the corresponding :aromatic carboxylic acids.

Another object of the invention is to provide a relatively cheap process for the conversion of such partially oxidized alkyl aromatic hydrocarbons into the corresponding aromatic carboxylic acids which process uses as the primary oxidizing agent air and/or oxygen and which rcsults in the production of substantially theoretical yields of the aromatic carboxylic acids.

A further object of the invention is to provide a continuous process for the conversion of `partial oxidation products of alkyl aromatic hydrocarbons into the corresponding aromatic carboxylic acids, which process permits the recovery ofthe acids as such or in the form of their anhydrides without the necessity for an acidification step involving the formation of inorganic salts as byproducts in the process.

A specific object of this invention is to provide a process for the conversion of ortho-toluic acid into phthalic acid and/ or phthalic anhydride in substantially theoretical yields, which process involves contacting the ortho-toluic acid with hydrogen suliide and an aqueous solution of sodium bisullite, which latter solution is constantly recycled in the process, to convert the ortho-toluic acid into disodium phthalate acidifying the disodium phthalate with sulfur dioxide produced by burning a portion of the hydrogen sulfide present in the system with air and/or oxygen, filtering thc acidied product at a relatively high temperature to remove free sulfur and again at a lower temperature to remove phthalic acid and dehydrating the phthalic acid produced as described to `form phthalic anhydride.

The above and related objects are accomplished by contacting the partial oxidation product of an alkyl aromatic hydrocarbon with hydrogen suliide and alkali metal or alkaline earth metal bisulte in the presence of a relatively large amount of water, the amount being suicient to completely' dissolve the bisulfite at filtration temperatures to be used in the recovery steps of the process, at temperatures between about 500 F. and 800 F., and pressures between about 1000 and 3000 p. s. i. g. for a time suicient to effect the oxidation which will generally be between about 0.3 and 1.5 or 2 hours. The metal bisulfitc and water used in the above contacting is a recycled stream from a later step of the process and once sufficient amounts of these materials are present in the system additional quantities need not be added except for make-up in case of loss. Alkali metal bisulfites, and particularly sodium bisulte, are preferred although the alkaline earth metal bisulftes, as for example, calcium, barium, strontium, Vand magnesium bisullites, may be employe'.

The above contacting resulting in oxidation of the partially oxidized alkyl aromatic hydrocarbon to the corresponding aromatic carboxylic acid may be effected in any pressure vessel which is corrosion resistant and capable of withstanding the temperatures and pressures involved, preferably this vessel will be equipped with means for agitation. The preferred vessel consists of a tubular or a manifolded tubular type vessel which insures that all of the charge remains in the vessel for the time necessary to complete the oxidation reaction. In such case turbulent flow in the tube or tubes provides sulhcient agitation and contacting.

Product leaving the oxidation vessel and consisting of the sodium or other alkali metal or alkaline earth metal salt of the aromatic carboxylic acid, hydrogen sulfide and sulfur is passed to a stripping vessel where hydrogen sulfide is removed. A portion of the hydrogen sulfide is re cycled to the oxidation step and the remainder is burned with air and/or oxygen to produce sulfur dioxide. The stripped oxidation product is then contacted with sulfur dioxide produced in the burning step. In this contacting vessel the metal salt of the organic carboxylic acid is converted into the corresponding acid, the metal being converted into the metal bisulte. Nitrogen and unused oxygen from the burning is vented from this contacting vessel.

The acidied product, consisting of water, aromatic carboxylic acid, metal bisulfite and free sulfur, is then passed to the recovery stage or stages of the process for separation of sulfur and the acid produced from the aqueous metal bisulfite solution, the latter solution being recycled to the oxidation step. The method of recovery of the carboxylic acid will depend upon the solubility characteristics of the acid. Where the acid produced is soluble in the metal bisulte solution at temperatures below the boiling point of this solution, as for example, 190-200u F.. the acidified product may be filtered to remove free sulfur and then cooled to effect crystallization of the aromatic acid which may then be filtered from the aqueous metal bisulte solution. In those instances in which the aromatic carboxylic acid is insoluble in a hot metal bisultite solution. the initial filtration removes both sulfur and aromatic acids, the sulfur being separated from the acid by means of solvent extraction, as for example, with carbon disulfide, or the like. Methods of separation are thus dependent upon the particular solubility characteristics of the acid or acids produced.

The following examples will serve to illustrate the methods of carrying out the process of this invention with respect to particular partially oxidized alkyl aromatic hydrocarbons.

As an example of this invention the process as applied to the oxidation of ortho-toluic acid is described herebelow and is illustrated in the accompanying tiow diagram. In this example, and referring particularly to the flow diagram, l mol of ortho-toluic acid, 3 mols of HzS and sufficient water to replace that lost in the system is introduced into an oxidation vessel together with 2 mols of NaHSOs, 3 mols of HzS and water (approximately 46 mols) which latter materials are recycled from later steps of the process. In the oxidation, referred to as step l, the reactants are maintained at 600 F. and 1800 p. s. i. g. for a period of one hour. The products of oxidation from step l, consisting of l mol of disodium phthalate, 5 mols of HzS, 3 atoms of sulfur, and water, are transferred through line A to a stripping vessel in which the hydrogen sulfide is stripped from the aqueous oxidation product in step 2. Stripping is effected at a temperature of approximately 240 F. Hydrogen sulfide (5 mols) is discharged through line B and separated into two streams, 3 mols of the hydrogen sulfide being returned through line C and line D to step l of the process and the remaining 2 mols of hydrogen sulfide being passed through line E to a burner where it is burned with air (3 mols O2) in step 3 to produce 2 mols of SO2 and water. This SO2 is transferred to and used in the acidification step, step 4, as will now be described.

The stripped product from step 2, consisting of 1 mol disodium phthalate, 3 atoms of sulfur and approximately 46 mols of water is discharged through line F into an acidifer where it is acidified. in step 4 by means of SO2 entering through line G. Since the SO2 entering through line G carries with it nitrogen and excess oxygen from the burning in step 3, these gases are vented from the acidifier. The acidification is carried out at a temperature of approximately F.

The acidified product from step 4 is transferred to a filter through line H where free sulfur (3 atoms) is removed and discarded. This filtration, step 5, is carried out at about 195 F. Filtrate from step 5 is discharged through line l into a second filter where filtration at a temperature between about 35 F. and 80 F. removes phthalic acid from the aqueous solution of NaHSOs in step 6. The filtrate, consisting of an aqueous solution of 2 mols of NaHSOa in approximately 46 mols of water, is discharged through line I in to line D and thence returned to the oxidation step. Phthalic acid separated by liltration in step 6 is discharged through line K into a dehydrator Where it is dehydrated in step 7 to produce l mol of phthalic anhydride.

It is to be noted that phthalic acid is slightly soluble in aqueous sodium bisulfite solution so that in actual practice the filtrate from step 6 recycled to step 1 of the process may contain some phthalic acid. This recycle of phthalic acid is constant for a given temperature of filtration in step 6 and therefore does not alter the conversion of orthotoluic acid to phthalic acid in the process. When low temperature filtration is employed, i. e., about 35 F., the amount of phthalic acid remaining in the filtrate is negligible.

The net reaction in the process is:

Thus it will be seen from the equation that the only chemicals consumed in converting the ortho-toluic acid are 3 mols of hydrogen sulfide and 3 mols of oxygen from air per mol of phthalic acid produced. The hydrogen sulfide is converted to sulfur, which in the form produced is a valuable by-product.

Although the above example relates to ortho-toluic acid, the process is applicable to mixtures of toluic acids and to metaor para-toluic acid. When mixtures are employed as feed stock, the product removed by filtration in step 5 will consist of sulfur together with any isoor terephthalic acid present in the acidified mixture. These phthalic acids can be separated from the sulfur by extraction with carbon disulfide or other method Well known in the art. Similarly when metaor para-toluic acid is used as feed the aromatic acid produced is removed in step 5 together with the sulfur and is then separated as mentioned above. The amount of oxygen and hydrogen sulfide required and the amount of sodium bisulfite recycled is the same as for ortho-toluic acid.

As would be readily understood by one skilled in the art, the above oxidation process could be started by charging to the oxidation vessel the reactants shown in the flow diagram as being used in step l. Thus, in addition to the material shown as feed, the material shown as recycle can be added to the system until the recycle streams are available. Alternatively, until the recycle streams are available they may be substituted for on start-up by an amount of hydrogen sulfide, sulfur and NaOH equivalent to the hydrogen sulfide and sodium bisultite shown as being recycled.

As a second example of the process of this invention, the process will be described as applying to the oxidation of acetophenone. In carrying out this oxidation, l mol of acetophenone, 2 mols of HzS and make-up water is charged to the oxidizer together with 2 mols off metal bisulfite, 4 mols of HzS and approximately 40 mols of water. The latter materials are recycled from later steps of the process. The oxidation in step l is effected at a pressure of approximately 1800 p. s. i. g. and a temperature of 600 F. `for a period of 1 hour. The products of oxidation consisting of l mol of sodium benzoate, 6 mols of HzS, 2 atoms of sulfur, 1 mol of carbonic acid and water are transferred to a stripping vessel from which hydrogen sulfide is removed. Stripping is effected at a temperature of [approximately 240 F. A portion of the hydrogen sulfide, approximately 4 mols, is recycled to the oxidation step and the remainder, 2 mols are burned with air to form sulfur dioxide. The sulfur dioxide produced is used to acidify the stripped oxidation product. Upon acidification the sodium benzoate is converted into benzoic acid `and carbon dioxide is released from solution. Carbon dioxide, nitrogen and unused oxygen are vented from the acidifier. The resulting product is filtered at a temperature of 190-200 F. to remove `free sulfur which amounts to 2 atoms and the resulting solution cooled to 40 F. and ltered to recover benzoic acid. Substantially complete removal of benzoic acid is effected `at this temperature. The filtrate consisting of sodium bisullite in water solution is recycled to the oxidation step.

The net reaction involved in this oxidation is illustrated by the following equation:

Thus, for each mol of acetophenone oxidized to benzoic acid, 2 mols of HzS and 3 mols of oxygen are used up and there is produced 2 `atoms of sulfur, 1 mol of CO2 and 1 mol of benzoic acid.

A third example of this invention illustrates the invention as related to the oxidation of 2-4-dimethyl benzoic acid to produce trimellitic acid. In this case l mol of 2-4-dimethyl benzoic acid, 3 mols of hydrogen sulfide and make-up water is charged to an oxidizer along with a recycle stream obtained from later steps of the process consisting of 3 mols of sodium bisulfite, 6 mols of hydrogen sulfide and approximately 60 mols of water. The oxidation is effected at approximately 600 F. and 1800 p, s. i. g. `for a period of 1 hour. The product from the oxidizer is stripped at a temperature of approximately 240 F. to remove hydrogen sulfide. The hydrogen sulfide stream is separated into two portions, 6 mols being returned to the oxidizer and 3 mols being burned with air to produce sulfur dioxide. The sulfur dioxide is returned to the stripped oxidized product to convert the trisodium salt of trimellitic acid contained therein into the acid, The neutralized product is filtered hot (i90-200 F.) `to remove sulfur and the filtrate cooled to 80 F., and extracted with diethyl ether to recover the trimellitic acid. The filtrate from this second filtration, consisting of aqueous sodium hisulfite, is recycled to the oxidation step of the process.

The net reaction involved in the above oxidation is illustrated by the following equation:

Thus, there is used up in the process 3 mols of hydrogen sulfide and 9/2 mols of oxygen per mol of trimellitic acid produced and there is produced in addition to the trimellitic acid, 3 atoms of sulfur.

The above specific examples illustrate the process of the invention which is applicable to the various partially oxidized alkyl aromatic hydrocarbons disclosed herein. The same conditions of temperature, pressure and time set forth in the examples and in the broader limits disclosed hereinabove are suitable for carrying out the Feed, Recycle, Mols Compound oxidized, 1 mol lIolSs HzS NaHSOs H2O (1) Benzyl alcohol 1 2 1 25 (2) Carboxy phenyldimethylcarbinol 4 8 4 0l) (3) Ethyl benzoic acid 3 6 3 75 The net reactions involved in the oxidations set forth in the above table may be illustrated by the following equations It will be noted that although sodium bisulte is shown in the above examples, the other alkali metal bisulfites, e. g., lithium and potassium bisulfites and the alkaline earth metal bisulfites, e. g., calcium, barium, strontium and magnesium bisultes if substituted for the sodium bisulfites will prouce similar results.

The amount of alkali or alkaline earth metal bisultite employed will be at least that amount necessary to provide sufficient metal to convert the acid or acids produced in the oxidation completely into their metal salts. Thus, it is found that if less than the equivalent amount of metal bisulfite is employed the oxidation reaction takes place more slowly and generally does not go to completion. Excess metal bisulte may be used but more than a few percent excess, as for example, 5-l0% is apparently of no value and merely increases the recycle load.

The amount of water to be employed is that amount necessary to maintain the metal bisulte in solution at the temperatures of filtration. Generally about 20-25 mols of water per mol of alkali metal bisulfite (or equivalent of alkaline earth metal bisulfite) will be employed, however, depending upon the solubility of the particular bisullite employed, as low as 15 mols of water per mol or equivalent of bisuliite may be employed. The upper limit of the amount of water employed does not appear to be critical, however, generally not more than 40 or 50 mols per mol of bisulfite is economically recycled in the process.

The above description and examples of my invention are illustrative of the invention but are not to be taken as limiting, particularly with respect to the partial oxidation products of alkyl aromatic hydrocarbons which may be oxidized using the process described since, as would be understood by those skilled in the art, alkyl aromatic hydrocarbons containing at least one atom of oxygen in at least one of the alkyl substituents of the aromatic nu` cleus may be oxidized following the teachings herein.

I claim:

1. A process for producing aromatic carboxylic acids which comprises, oxidizing a partial oxidation product of an alkyl aromatic hydrocarbon by contacting said partial oxidation product with an aqueous mixture of a metal bisulte selected from the class consisting of alkali and alkaline earth metal bisulfites and hydrogen sulfide at a temperature between about 500 F. and about 800 F. and a pressure between about l000 and about 3000 p. s. i. g., stripping the aqueous oxidized mixture to remove hydrogen sulde, returning a portion of said removed hydrogen sulfide to the oxidizing step, burning the remainder of said removed hydrogen sulfide, contacting the stripped aqueous oxidized mixture with the products of said burning, separating an aqueous solution of said metal bisulfite from the product of said last named coutacting and returning said aqueous solution to the oxidizing step.

2. A continuous process for the production o-f aromatic carboxylic acids having the carboxyl carbon atoms directly attached to a nuclear carbon atom which comprises, oxidizing a partial oxidation product of an alkyl aromatic hydrocarbon with water, hydrogen sulfide and a metal bisulfite selected from the class consisting of alkali and alkaline earth metal bisulfites at a temperature between about 500 F. and about 800 F. and a pressure between about 1000 and about 3000 p. s. i. g., stripping the resulting aqueous oxidized mixture to remove hydrogen sulfide, returning a portion of said removed hydrogen sulfide to the oxidization step, burning the remainder of said removed hydrogen sulfide with air, contacting the stripped aqueous oxidized mixture with the products of said burni ing, separating free sulfur and aromatic carboxylic acids from the product of said contacting, and returning the remainder of the product of said contacting to the oxidation step.

3. A process according to claim 2 in which the metal of said metal bisulfite is an alkali metal.

4. A process according to claim 3 in which said alkali metal is sodium.

5. A process according to claim 2 in which said partial oxidation product of an alkyl aromatic hydrocarbon is an alkyl benzoic acid.

6. A process according to claim 5 in which said alkyl benzoic acid is a toluic acid.

7. A process according to claim 5 in which said alkyl benzoic acid is ortho-toluic acid.

8. A process according to claim 5 in which said alkyl benzoic acid is a dimethyl benzoic acid.

9. A process according to claim 2 in which said partial oxidation product of an alkyl aromatic hydrocarbon is acetophenone.

l0. A method of producing phthalic acids from a mix ture of isomeric toluic acids which comprises oxidizing said mixture of toluic acids by contacting said mixture with water, hydrogen sulfide and a metal sulfide selected from the class consisting of alkali and alkaline earth metal bisulfites at a temperature between about 500 F. and about 800 F. and a pressure between about 1000 and about 3000 p. s. i. g., stripping the aqueous oxidized mixture to remove hydrogen sulfide, returning a portion of said removed hydrogen sulfide to the oxidation step, burning the remainder of said removed hydrogen sulfide with air to produce sulfur dioxide, acidifying the stripped aqueous oxidized mixture with said sulfur dioxide, filtering the acidified mixture at a temperature of approximately 195 F. to remove sulfur and phthalic acids other than ortho-phthalic acid, filtering the filtrate from the first filtration at a temperature between about 35 F. and about F. to recover ortho-phthalic acid and returning the filtrate from said last named filtration to the oxidation step.

l1. A method for producing phthalic anhydride which comprises reacting ortho-toluic acid with sodium bisulfite and hydrogen sulfide in the presence of watcr at a ternperature of about 600 F. and a pressure of about 1800 p. s. i. g., stripping the reaction product to remove hydrogen sulfide, returning a portion of said removed hydrogen sulfide to said reacting step and burning the remainder of said hydrogen sulfide with air to produce sulfur dioxide, contacting said stripped reaction product with said sulfur dioxide, filtering the resulting product at about F. to remove free sulfur, refiltering said mixture at a temperature between about 35 F. and about 80 F. to remove phthalic acid, recycling the filtrate from said reltering to said reacting step and dehydrating the phthalic acid to produce phthalic anhydride.

References Cited in the file ot this patent UNITED STATES PATENTS 

1. A PROCESS FOR PRODUCING AROMATIC CARBOXYLIC ACIDS WHICH COMPRISES, OXIDIZING A PARTIAL OXIDATION PRODUCT OF AN ALKYL AROMATIC HYDROCARBON BY CONTACTING SAID PARTIAL OXIDATION PRODUCT WITH AN AQUEOUS MIXTURE FO A METAL BISULFITE SELECTED FROM THE CLASS CONSISTING OF ALKALI AND ALKALINE EARTH METAL BISULFITES AND HYDROGEN SULFIDE AT A TEMPERATURE BERTWEEN ABOUT 500*F. AND ABOUT 800*F. AND A PRESSURE BETWEEN ABOUT 1000 AND ABOUT 3000 P.S.I.G., STRIPPING THE AQUEOUS OXIDIZED MIXTURE TO REMOVE HYDROGEN SULFIDE, RETURNING A PORTION OF SAID REMOVED HYDROGEN SULFIDE TO THE OXIDIZING STEP, BURNING THE REMAINDER OF SAID REMOVED HYDROGEN SULFIDE, CONTACTING THE STRIPPED AQUEOUS OXIDIZED MIXTURE WITH THE PRODUCTS OF SAID BURNING, SEPERATING AN AQUEOUS SOLUTION OF SAID METALIC BISULFITE FROM THE PRODUCT OF SAID LAST NAMED CONTACTING AND RETURNING SAID AQUEOUS SOLUTION TO THE OXIDIZING STEP. 